Increased expression of IAP elements in tumor cells compared with normal cells is accompanied by extensive hypomethylation of IAP sequences. We have used oligonucleotide probes based on sequences in the LTRs of expressed IAP elements to examine the methylation state of subsets of the endogenous IAP proviral sequences, and have found that multiple common IAP loci are hypomethylated in B-cell tumors. However, not all IAP elements are hypomethylated even when their LTR sequences are very similar, suggesting that the methylation state of the proviral DNA is determined by their position in the genome. We have now mapped a number of the commonly hypomethylated loci in B-cell tumors. These include regions on chromosome 2 (2 loci), 4, 5 (2 loci), 7, and 17. Thus far extensive attempts to isolate DNA regions flanking these loci have been unsuccessful. A number of other loci that have been isolated have proven to be non-polymorphic between mouse strains and have shown no variations in methylation state. We had shown that the pattern of hypomethylated IAP loci in normal plasma cells (terminally differentiated B-cells from an IL6 transgenic mouse) was more like that in normal LPS-stimulated B-cells than that in plasmacytomas. We have now confirmed by Northern blot analysis and PT-PCR that the IAP elements expressed in plasma cells are of the lymphocyte specific type rather than of the tumor type. These results support earlier studies showing a relationship between hypomethylation of IAP elements and their expression.